It is known that the strength of steel starts to fall when the temperature of the steel exceeds 500° C. or so, and falls to zero at about 1000° C. or so. As building fires may exceed these temperatures, it is clearly desirable that structural beams made of steel retain sufficient strength to avoid deformation for a period which is sufficiently long for, for example, the building to be evacuated. Typical fire protection periods for structural beams, particularly floor supporting beams, vary from 30 to 120 minutes. The fire resistant qualities of the beam can be increased by increasing the physical characteristics, that is the physical dimensions, of the beam and/or by insulating the beam such that in the event of a fire, the rate of temperature rise of the beam will be reduced to provide the required length of fire resistance. It is known, for example, to provide a suitable fire resistant cladding, which is built around the beam on site. This however actually requires additional on-site work, which may extend the time required to commission a building, with attendant financial cost.
It is also known to apply a fire protection material to a beam, which is subject to an intumescent reaction when heated or in the presence of fire. When heated, the material undergoes an interaction between its components which causes the material to form a char, the thickness of which is up to 50 times that of the original coating of the fire protection material. The char has insulating properties and so decreases the rate of temperature rise in the steel element to which it is applied. Hence, a structural beam may be supplied with desired fire resistant values without necessarily having to increase the physical dimensions of the beam.
Typically, intumescent fire protection material is applied as a coating to a structural beam by being supplied as a spray. The resulting coating has a thickness typically in the range of 250 to 2200 microns, and thicker if need be. The spray may be applied on site or off site. The advantage of applying the coating off site is that a fully finished structural beam is supplied to the construction site which reduces the work required on site, and hence shortens the construction period and reduces the cost.
Conventionally, when assessing the thickness of fire protection material required, an engineer will consult an appropriate reference book, such as “Fire Protection for Structural Steel in Buildings” published by the Association of Specialist Fire Protection and the Steel Construction Institute. This will suggest an appropriate thickness of intumescent coating to be applied to a beam depending on the section factor of the beam, that is its perimeter distance divided by its area, and the length of time for which fire resistance is required.
There are difficulties in this approach in that it does not fully take account of cellular beams or other structural beams provided with apertures, and it does not consider parameters such as cell spacing or web slenderness ratio.